Making Copies of Copies
Although it surprises many people—it certainly surprised us—the CDs you make in your burner are not necessarily exact copies of the source CD. For audio CDs, this usually isn’t a problem. An occasional dropped or flipped bit will probably be inaudible. But for data CDs, it can be a major problem. We found this out while playing with a new burner. We duped the original FrontPage CD, and were able to install FrontPage from the dupe. But when we duped the dupe and then tried to install from the second-generation copy, Setup returned file read errors. Doing a binary compare on those files told the sad truth: the files were identical on the original CD and the first-generation copy, but differed on the second-generation copy.
Remember that CDs depend heavily on error-correction and detection code. All CD formats use 3,234-byte physical sectors, which allocate 2,352 bytes to data and the remainder to control and error correction data. Audio CDs use the entire 2,352 bytes for audio data. Yellow Book Mode 1 (data) CDs use only 2,048 bytes for data and allocate the remaining 304 bytes to another ECC layer. The problem arises because of the different ways that CD burners and software read and write Mode 1 data:
- Raw mode
The drive reads the entire 2,352 byte sector as a raw bit stream and writes that raw data to the destination device. Any data read errors in either the data segment or the ECC segment are written literally to the destination. Working in this mode, generational ...